Structural and non-structural intensity measures for the assessment of base-isolated structures subjected to pulse-like near-fault earthquakes

The objective of the present work is to improve selection procedures for intensity measures (IM) to be used for scaling near-fault earthquakes, in order to minimize the variability in the prediction of engineering demand parameters (EDPs). To this end, the predictive capability of nine spectral IMs, among the most commonly used in the literature, is investigated with reference to three EDPs evaluated for base-isolated structures subjected to near-fault earthquakes, which can be characterized by pulse-type motions in the horizontal direction and significant vertical component. A six-storey reinforced concrete (r.c.) framed building originally designed as fixed-base, in compliance with a former Italian seismic code for a medium-risk zone, is retrofitted by means of friction pendulum bearings (FPBs), to attain performance levels imposed by the current Italian code in a high risk seismic zone. The nonlinear dynamic analysis is carried out by a lumped plasticity model for the r.c. frame members, including a 26-flat surface axial load -biaxial bending moment elastic domain at the end cross sections, and a nonlinear force-displacement law for the FPBs, that consider variable axial load combined with friction coefficient as function of the sliding velocity. Two sets of accelerograms are considered, consisting of near-fault records with significant horizontal pulses, selected by using a pulse index, and significant vertical component, selected by the ratio between the peak of the vertical and the horizontal ground acceleration.